Data migration service FAQ

Below are common questions about running a migration with the data migration service.

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Before you migrate data

What is the data migration service?

The data migration service is a free product offered by Google. It helps you easily import your email, calendar, or contact data into your G Suite account. All eligible data is copied during the import, not moved/deleted, and the data in your source mailbox remains after a migration completes. Learn more

Is there a maximum number of users I can migrate?

The data migration service migrates content for a maximum of 100 users. For migrations with more users or complex requirements, use the G Suite migration product matrix to pick the right product for your source data.

Is there a limit to how many emails I can import?

By default, Google gives you 15 GB of storage space for everything associated with your Google Account. Your storage limit is shared across Google Drive, Gmail, and Google Photos.

You cannot import more data into Gmail than your existing storage space allotment. Messages and attachments, including items in your Spam and Trash folders, count toward your storage limit.

Important: When your account reaches its storage limit, or if you downgrade your account and your files now exceed your storage space, you cannot send or receive messages in Gmail.

For details on storage space, go to Clear Google Drive space & increase storage.

How do I ensure that email messages are RFC-compliant?

Messages must be RFC-compliant to be migrated with the data migration service. If they aren't, you’ll receive the error message “Our system has detected that this message is not RFC 2822 compliant" or the message might be migrated with the wrong date.

Here are some common reasons why messages aren't RFC-compliant:

  • The From field can’t be empty and must only have a single address.
  • The domain in the From field can't be malformed.
  • The Date field must have 4 digits for the year, not 2.
  • The time of day field must be in the range 00:00:00 through 23:59:60.

If you can edit the message in the source mailbox, you might be able to correct the message headers and remigrate. If not, you should migrate the messages some other way (for example, by forwarding them to the new account using your email client). For information about correcting the messages, contact your legacy account provider.

Can resellers migrate data for a customer?

No, resellers can't migrate data for their customer accounts.

Can I migrate IMAP email folders that are shared or public?

The data migration service does not support migrating shared or public IMAP folders.

Do I run a migration before or after I add the domain?

A migration is run after the G Suite domain is added. For more details, go to Set up your new G Suite domain.

Running a migration

How long will a migration take?

The duration depends on several factors, such as:

  • The number of users you’re migrating.
  • The migration start date you select—the longer the time frame, the longer a migration takes.
  • The number and size of items being migrated.
  • The performance of the legacy server you’re migrating from.
  • The network's data transfer rate.
  • Latency between Google and the legacy server.
What happens to the data in the source mailbox? Does it remain or is it deleted?

The data migration service does not remove data from the source mailbox, nor does it remove the original source mailbox. All eligible data is copied during a migration, not moved/deleted, and the data in your source mailbox remains after a migration.

Note: The data migration service doesn't migrate messages, including attachments, larger than 25 MB. For details, go to Send attachments with your Gmail message.

Can I migrate all email messages in one migration?

Yes, you can choose a custom start date to specify the entire period on your legacy account. If you have many years or multiple gigabytes of data, we don't recommend migrating all email messages in a single migration. It takes a long time to complete a migration of this size. For details on custom start dates, go to Specify a migration start date.

Can I set an end date to specify a date range?

No. The data migration service migrates email or calendar events from the start date entered to the current day, beginning with the most recently dated events or messages. For details, go to Specify a migration start date.

Note: You cannot change the migration settings (including the start date) once the migration has started.

What happens to email received in the legacy account when a migration is running?

If you’re migrating from Microsoft Exchange or Exchange Online (Office 365), any new messages delivered into the legacy account during the migration are transferred until you exit the migration. If you’re migrating from any other platform, any new messages delivered to the legacy account during the migration are not migrated. For more details, go to How do I migrate newly received emails (those received after the migration completes)?

Users can set up automatic forwarding on their legacy account to ensure any new email messages are sent to their new Gmail account. Consult the documentation for your legacy account for instructions on how to do this. If you’ve migrated from Gmail or another G Suite account, go to Automatically forward Gmail messages to another account.

If your MX records aren’t pointing to Google during the migration period, you can create automatic forwarding to a test domain alias. For details, go to About test domain aliases.

Can I migrate from multiple sources to one user account?

No. The data migration service is designed to allow only one migration at a time to a specific user.

We don't recommend having multiple concurrent API requests migrating data into single account.

During a migration

How do I pause, cancel, or exit a migration?

When you use the data migration service, you can pause, restart, cancel, or exit a migration. Learn more

To change the migration settings during a running migration, you must first cancel the migration. For details, go to Cancel a migration.

Why does my migration seem like it's stuck at 99%?

You’ll see 99% when all email is migrated. After everything is migrated, the data migration service applies any labels to the migrated email, which can take time. When the labels are applied, the migration is now complete (100%).

You might also get this issue if the estimated number of emails to migrate exceeds the actual number of messages. The migration will report 99% until the migration completes. This process might take some time.

How do I know the migration is complete?

When the migration finishes, the word Complete appears in the Status column for a user in the Migrations table. To view metrics about a migration, point to Complete in the Status column. A tooltip displays the following statistics:

  • Items successfully migrated
  • Items that failed to migrate
  • Total items discovered

Another way to verify if a migration completed successfully is to check the migration report for errors. For more details, go to Monitor a migration.

Note: When the migration is complete, you should exit the migration. A migration report is automatically sent to all super administrators of the account and the migration role account. To exit a completed migration, go to Exit a completed migration.

Rerunning a migration

If I rerun a migration, will email messages be duplicated?

No. If you exit and rerun a migration, any messages already migrated won't be duplicated.

The data migration service checks if each message is already in the destination mailbox before trying to migrate it. It skips messages that are already there, instead of downloading them again from the source mailbox.

If I rerun a migration, will calendar events or secondary calendars be duplicated?

No. If you exit and rerun a migration, any calendar events or secondary calendars that are already migrated won't be duplicated.

However, if a user changes the name of a secondary calendar between migrations, that secondary calendar is duplicated when it's remigrated.

After a migration

Once I migrate data, how do I migrate data from an earlier start date?

If you previously migrated data from a specific start date, and later decide you want to migrate data from an earlier start date, you must exit the migration and complete a full data migration using the earlier date. The data migration service automatically checks every message before migrating it, identifies duplicate messages, and eliminates them. It does this check regardless of how the migration is run, or how the message was previously inserted the account. For details, go to Specify a migration start date.

How do I migrate newly received emails (those received after the migration completes)?

If you need to fetch emails received on the source mailbox after the migration to the destination mailbox completes, you must start another data migration. For more details, go to What happens to email received in the legacy account when a migration is running?

Can I roll back a completed migration?

There’s no option to reverse or roll back a completed migration. This feature does not exist in the data migration service.

If you need to complete another migration, you can delete all users and start again.

How can I find the migrated emails in my account?

When migrating from the source mailbox to a destination mailbox with existing emails, no custom label identifying the migrated emails is added.

In Gmail, messages show the wrong date

Migrated messages might show the migration time and date, rather than the time and date of the original message.

This most likely occurred because the date header of the original message wasn't compliant with RFC 5322. If a message has a date header that isn't formatted correctly, Gmail applies the migration time and date to the message.

Why is there a difference between the total number of messages following a migration?

Following a migration, you might notice the number of email messages in the source mailbox exceeds the number of messages migrated to Gmail.

This discrepancy might be due to the difference between folders in Exchange and IMAP mail servers and labels in Gmail. In Exchange and on IMAP mail servers, when you assign a message to more than one folder, the message is duplicated. Following a migration, the duplications are removed in Gmail and different labels are applied instead.

To verify that all messages have been migrated, check the logs for any errors. For details, go to Monitor a migration.

If you think you’re missing some messages, check whether:

  • The message, including attachments, is not bigger than 25 MB. You can’t migrate messages bigger than 25 MB. For details, go to Send attachments with your Gmail message.
  • Gmail is not blocking an attachment to the message. Gmail blocks certain types of attachments, such as executable files. For details, go to File types blocked in Gmail.
  • The message is in a folder or within a date range that’s part of the migration.

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